Method for desalinating and demineralizing solutions containing acids and/or metal salts

ABSTRACT

A process for the desalination and demineralization of solutions conmining acids and/or metal salts comprising a number of steps after introducing a solution to be treated. Initially, chemical agents are added to precipitate predetermined salts. Afterward clarification occurs. An inhibitor is then introduced for inhibiting further precipitation of the salts. The process proceeds by concentrating the salts in 5%-80% of tile flow of the solution to be treated into a supersaturated concentrate to form a concentrated fraction, the remaining fraction being a demineralized permeate. There is then removal of the effect of the precipitation inhibitor allowing precipitation of crystallizable supersaturated salts in the concentrate.

FIELD OF THE INVENTION

The present invention relates to the qualitative and quantitativereduction of the content of precipitable ions in aqueous or othersolutions and to the use of these solutions.

The invention more particularly applies to the desalination and to thedemineralization of water, in particular of potable water or of processwater, for the purpose of satisfying potability restrictions orrestrictions related to the use of water and to the recycling of wastewater in industry.

BACKGROUND OF THE INVENTION

Various desalination or demineralization or crystallization processesare currently known. The main processes will be restated hereinbelowwith the disadvantages which are associated with them:

the precipitation of salts of very slight solubility which are separatedfrom the treated water by settling or filtration. Thus, French PatentFR-A-2,339,575 discloses a process and a plant for the treatment ofwater by crystalline precipitation and settling which consists inproducing a homogeneous dispersion of water to be treated, of reagentsnecessary for the crystalline precipitation and of sludges separatedfrom the water to be treated and in settling this dispersion, thecrystalline precipitation reaction being completed during the settling.The disadvantage of this known technique lies in the use of reagentswhich can be very expensive for the purpose of reducing their contentsof salts of very slight solubility below their solubility;

percolation through a bed of ion-exchange resins which makes it possibleto replace all or part of the ions by other ions. This solution exhibitsthe disadvantage of high consumption of chemical reagents and that ofthe production of saline, indeed highly saline, regeneration eluates;

filtration through reverse osmosis or nanofiltration membranes which areselective with respect to certain salts and which concentrate them in afraction of the treated flow. The disadvantage of this technique lies inthe production of a filtration concentrate which represents asignificant fraction of the flow of the water to be treated (of theorder of 10 to 70%, depending on the initial salinity and the degree ofconversion adopted);

evaporation, which produces demineralized water by condensation of thevapor produced and which concentrates the salinity into a brine. Thissolution leads to very high energy consumption and the production of asaline brine which has to be either discharged or crystallized, thelatter stage resulting in a very high investment cost;

electrodialysis, involving ion-exchange membranes which make it possibleto extract the inorganic salts from the solutions and to concentratethem into a brine, the purified water flow being separated from thebrine flow by the said ion-exchange membranes.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides for the introduction of a process fordesalination or demineralization with the possibility of crystallizationwhich does not exhibit the disadvantages of the currently knownsolutions briefly restated hereinabove.

The desalination and demineralization process according to the presentinvention mainly relates to solutes, the solutions of which can besupersaturated by addition of chemical reagents.

The solutes can be, inter alia, inorganic or organic acids and theirsalts.

Consequently, the subject-matter of the present invention is a processfor the desalination, demineralization and optionally crystallization ofsolutions containing acids and/or metal salts, characterized in that itcomprises the following successive stages:

a) conditioning the solutions, clarified before-hand, using a chemicalreagent which is an inhibitor of the precipitation of salts, optionallyfollowed by a filtration;

b) concentrating the salts into a supersaturated brine representing afraction of the flow of between 5 and 80% of the flow of the solution tobe treated;

c) in this brine, reducing, indeed removing, the effect of theprecipitation inhibitor and precipitating the crystallizablesupersaturated salts in the concentrated fraction.

Still other objects and advantages of the present invention will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only the preferredembodiments of the invention, simply by way of illustration of the bestmode contemplated of carrying out the invention. As will be realized,the invention is capable of other and different embodiments, and itsseveral details are capable of modifications in various obviousrespects, without departing from the invention. Accordingly, thedrawings and description are to be regarded as illustrative in natureand not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of the present invention will be moreclearly understood when considered in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a flowchart of a first embodiment of the invention; and

FIG. 2 is a flowchart of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to an embodiment of the process according to the invention asdefined hereinabove, an additional preliminary stage of neutralizationcan be provided consisting of a chemical modification of the solutionfor the purpose of reaching the solubility product of the ion or ions tobe removed. According to the invention, this additional stage is carriedout by introducing reagents, for example addition of calcium in the formof calcium chloride or calcium hydroxide, in order to precipitate thesulphates or fluorides. According to the invention, this optionaladditional preliminary stage of neutralization may or may not befollowed by a clarification stage.

According to the invention, a final additional stage can be providedconsisting of recycling the concentrated fraction at the head of theclarification stage, before or after the stage ofprecipitation/crystallization of the supersaturated salts.

According to the present invention, the inhibitors of the precipitationof the salts removed are chosen from the group comprising phosphonates,polyacrylates and polycarboxylates.

According to an embodiment of the process according to the invention,the stage of demineralization of the solution and of concentration ofthe salts is preferably carried out by filtration through reverseosmosis or nanofiltration membranes, which are selective or otherwisewith respect to the ions to be removed, or by evaporation and/orcrystallization and/or dialysis and/or electrodialysis and/orelectro-electrodialysis.

According to the invention, the reduction, indeed removal, of the effectof the precipitation inhibitor can be carried out using a metal with avalency which may or may not depend on the quality of the precipitationinhibitor, in particular a trivalent metal, and/or by increasing the pHand/or by increasing the temperature and/or by the action of a chemicaloxidizing agent and/or by the action of any other chemical additive,inorganic or organic, which makes it possible to neutralize, in all orin part, the activity of the precipitation inhibitor.

As a non-limiting example, results obtained experimentally during theimplementation of the process according to the invention on asemi-industrial scale have been given hereinbelow. The effluents treatedduring this operation were waters originating from the mining industry.In this implementational example, which is without any limiting nature,the following steps were combined: conditioning the waters, saturatedwith calcium sulphate, with a polyacrylate, filtration through ananofiltration membrane and destruction of the polyacrylate with aferric iron salt.

    ______________________________________                                        The starting raw water exhibited the following characteristics:               ______________________________________                                        conductivity:        7 μS/cm                                               calcium:             725 mg/l                                                 sulphates:           2200 mg/l                                                turbidity:           0.25 NTU                                                 pH:                  7.9                                                      ______________________________________                                    

This water was conditioned using an inhibitor of precipitation ofcalcium sulphate composed of 2.5 mg/l of polyacrylate.

The water, thus conditioned, was subsequently filtered through ananofiltration membrane (operating pressure: 7 bar; degree ofconversion: 66%).

    ______________________________________                                        The characteristics of the permeate and of the                                concentrate obtained were respectively as follows:                            ______________________________________                                        permeate:                                                                     flow:             66% of the feed flow                                        conductivity:     5 μS/cm                                                  sulphates:        100 mg/l                                                    turbidity:        0.2 NTU                                                     concentrate:                                                                  flow:             34% of the feed flow                                        conductivity:     12.5 μS/cm                                               calcium:          2150 mg/l                                                   sulphates:        6600 mg/l                                                   turbidity:        0.35 NTU                                                    ______________________________________                                    

The reduction, indeed elimination, of the inhibiting effect onprecipitation of the polyacrylate was achieved using 16 mg/l of ferriciron in the concentrate. After self-crystallizing for one hour, thecontent of soluble polymer was below the detection threshold of theanalytical method, i.e. 0.2 mg/l. The content of soluble calciumsulphate was in the region, to within about 5%, of the theoreticalsolubility of calcium sulphate.

Two implementational examples of plants employing the process accordingto the present invention have been represented diagrammatically in theappended drawings, as non-limiting illustration.

FIG. 1 refers to a closed-circuit desulphatation plant with recycling ofthe concentrate from the nanofiltration stage.

It is seen, in this FIG. 1, that the water to be treated is subjectedbeforehand in 1 to a neutralization-precipitation treatment withintroduction of calcium compounds (Ca(OH)₂, CaCl₂), followed byflocculation in 2 and then by settling in 3 (in this implementationalexample, a lamellar settling tank can be used). In this device, aportion of the sludges is recycled at the head of the plant, theremainder being subjected to a dewatering stage as shown in this FIG. 1.The clarified water is conditioned by the inhibitor of precipitation ofthe calcium sulphate, before being filtered, for example, through a sandfilter 4. The stage of desulphatation of the water and of concentrationof the sulphate salts is here carried out through a nanofiltrationmembrane 5. The permeate is subsequently discharged and the concentrateis recycled at the head of the process, as has been shown in FIG. 1. Thecrystallization of the supersaturated salts in the concentrate ispromoted by virtue of mixing the crystals from the settling tank withthe concentrate, for example via a jet pump, thus using the availableenergy on the concentrate.

FIG. 2 refers to a desulphatation plant operating in an open and/orsemi-open circuit, with independent treatment and total or partialdischarge of the concentrate from the nanofiltration.

The same plant is found in this FIG. 2 as that described above withreference to FIG. 1, the difference being that the concentrate from thenanofiltration is subjected to crystallization-coagulation treatment in7, then flocculation in 8 and finally settling in 9, the water saturatedwith calcium sulphate subsequently being discharged.

Mention may be made, among the advantages of the process which is thesubject-matter of the present invention, of the following in particular:

low operating costs with respect to reagents (calcium salts, calciumchloride or calcium hydroxide, inhibitor of precipitation of salts,metal salt),

removal of anions, such as sulphates, phosphates, carbonates orfluorides, and of silica in the form of precipitates of metal salts orof hydroxides which can be easily dewatered (this list not beinglimiting),

removal of cations, such as calcium, strontium, barium and other heavymetals, in the form of simple or complex metal salts or in the form ofhydroxides,

possibility of producing a flow of treated water (degree of conversion)in the region of 100% of the flow of raw water (the difference arisingfrom the water discharged with the metal precipitates).

It remains, of course, that the present invention is not limited to theimplementational or production examples described and/or representedhere but that it encompasses all the alternative forms thereof.

We claim:
 1. A process for the desalination and demineralization ofsolutions containing acids and/or metal salts and comprising thesteps:introducing a solution to be treated; introducing an inhibitor forinhibiting precipitation of predetermined salts in the solution;concentrating the salts in 5%-80% of the flow of the solution to betreated into a supersaturated concentrate to form a concentratedfraction, the remaining fraction being a demineralized permeate;removing the effect of the precipitation inhibitor; precipitatingcrystallizable supersaturated salts in the concentrate; and introducingreagents into the solution, prior to the introduction of an inhibitor,that precipitate the salts by affecting the solubility product ofsolutes to be precipitated.
 2. The process according to claim 1 furthercomprising clarifying the solution after neutralization.
 3. A processfor the desalination and demineralization of solutions containing acidsand/or metal salts and comprising the steps:introducing a solution to betreated; introducing an inhibitor for inhibiting precipitation ofpredetermined salts in the solution; concentrating the salts in 5%-80%of the flow of the solution to be treated into a supersaturatedconcentrate to form a concentrated fraction, the remaining fractionbeing a demineralized permeate; removing the effect of the precipitationinhibitor; precipitating crystallizable supersaturated salts in theconcentrate; wherein the step of removing the effect of theprecipitation inhibitor comprises the introduction of a metal.
 4. Aprocess for the desalination and demineralization of solutionscontaining acids and/or metal salts and comprising the steps:introducinga solution to be treated; introducing an inhibitor for inhibitingprecipitation of predetermined salts in the solution; concentrating thesalts in 5%-80% of the flow of the solution to be treated into asupersaturated concentrate to form a concentrated fraction, theremaining fraction being a demineralized permeate; removing the effectof the precipitation inhibitor; precipitating crystallizablesupersaturated salts in the concentrate; wherein the step of removingthe effect of the precipitation inhibitor further comprises introducinga trivalent metal.
 5. A process for the desalination anddemineralization of solutions containing acids and/or metal salts andcomprising the steps:introducing a solution to be treated; introducingan inhibitor or inhibiting precipitation of predetermined salts in thesolution; concentrating the salts in 5%-80% of the flow of the solutionto be treated into a supersaturated concentrate to form a concentratedfraction, the remaining fraction being a demineralized permeate;removing the effect of the precipitation inhibitor; precipitatingcrystallizable supersaturated salts in the concentrate; wherein the stepof removing the effect of the precipitation inhibitor further comprisesintroducing a chemical oxidizing agent.
 6. A process for thedesalination and demineralization of solutions containing acids and/ormetal salts and comprising the steps:introducing a solution to betreated; introducing an inhibitor for inhibiting precipitation ofpredetermined salts in the solution; concentrating the salts in 5%-80%of the flow of the solution to be treated into a supersaturatedconcentrate to form a concentrated fraction, the remaining fractionbeing a demineralized permeate; removing the effect of the precipitationinhibitor; precipitating crystallizable supersaturated salts in theconcentrate; wherein the step of removing the effect of theprecipitation inhibitor further comprises introducing a chemicaladditive that is capable of neutralizing activity of the precipitationinhibitor.